Variable reluctance or switched reluctance motors conventionally have poles or teeth on both the stator and the rotor (i.e. they are doubly salient). There are windings on the stator but no windings on the rotor. Each pair of diametrically opposite stator windings is connected in series to form one phase of the switched reluctance motor.
Torque is produced by switching current on in each phase in a predetermined sequence so that a magnetic force of attraction results between the rotor and stator poles that are approaching each other. The current is switched off in each pair of windings at the commutation point before the rotor poles nearest the stator poles of that pair of windings rotate past the aligned position, otherwise the magnetic force of attraction will produce a negative or braking torque.
The torque developed is independent of current direction. Therefore, unidirectional current pulses synchronized with rotor movement can be generated in a converter using a single unidirectional current switching element such as a thyristor or transistor in each leg of the converter, and supplied to the corresponding phase of the motor.
Each time a phase of the switched reluctance motor is switched on by closing a switch in a converter, current flows in the pair of stator windings, providing energy from a DC supply to the motor. The energy drawn from the supply is converted partly into mechanical energy, by causing the rotor to rotate towards a minimum reluctance configuration, and partly into a magnetic field. When the switch is opened, part of the stored magnetic energy is converted to mechanical output and the remainder of the energy is preferably returned to the DC source.
The converter must switch the phase currents on and off in precise synchronism with the rotor position. This "shaft-position switching" is normally accomplished using a shaft position sensor by referencing the switching of the transistors in each converter leg to a set of pulses derived from the shaft position sensor. One example of a shaft position sensor is a fixed light source and a fixed light detector on opposite sides of a slotted disc connected to the shaft which optically interrupts the light beam between the source and the detector in accordance with the position of the shaft. The shaft position sensor is undesirable in small motors because of its cost, and in both large and small motors because of its space requirement and the vulnerability of the signal wires that must run between the motor and the electronic power converter.
It is known to run variable reluctance stepping motors without a shaft position sensor and without loss of steps by a method wherein very wide current pulses are provided by the converter without reference to the rotor position. The stepping motor thus operates at a very high torque margin so that torque transients do not cause a loss of steps. While the method described for operating stepping motors has high stability, the high torque margin is obtained at the expense of efficiency.